1. Field of the Invention
This invention is directed to a method and to a circuit arrangement for the condition control of a resonant circuit which successively assumes a charging and a discharging condition which, in a power pack formed on the principle of a resonant converter, forms the primary circuit of a transformer which has its output voltage at the secondary side monitored by a regulating means which initiates the beginning of every charging condition dependening on the height of the output voltage at the secondary side.
2. Description of the Prior Art
In past years, electronic devices have become smaller, more compact and lighter in weight despite their increasing complexity. This development is largely based on the increasing utilization of integrated circuits. Only the power pack remains unaffected by this development since the volume and weight can only be reduced to an extremely slight degree using the traditional longitudinal regulator principle.
Significant advances in this respect could only be achieved by the constructing of the power pack as a switched power pack. As a result of the development of new components, particularly fast-switching transistors, it is presently possible to construct switched power packs which have high reliability and economic advantage. They are not only smaller and lighter but also have a higher efficiency than conventional power packs.
The principal of the switched power pack is essentially based on chopping an input DC voltage by using a fast switch, of transforming the rectangular voltage which is obtained with a transformer and again rectifying the output voltage of the transformer. The output voltage is stabilized with a control which either influences the pulse-duty factor or the frequency of the switching event.
As is known from the publication, "Schaltnetzteile", by Joachim Wuestehub et al, Expert Verlag 1979, such DC converters, based on their functional principle, operate either as a blocking oscillator converter, a forward converter or a push-pull converter.
The output power obtainable with these converters is principally limited only by the load which can be carried by the switching transistors.
The volume and weight of a switched power pack, however, are essentially determined by the inductive components which are thus relevant to the power density, ie. to the relationship between the maximum power that can be output and the structural size or the weight of the power pack.
In order to increase the power density, then the structural sizes of the inductances must be reduced and their efficiency must be retained. This, however, can only be achieved by increasing the switching frequency.
By merely increasing the switching frequency to above about 80 KHz results in a noticeable reduction of efficiency in a switched power pack. The reason for this is the extremely high rise rates of the currents and voltages in switched power packs operating in accord with traditional principles. Parasitic effects such as the skin effect also take effect at higher frequencies.
Further losses contributing to a reduction in the efficiency occur at the secondary side of the transformer due to the rectifier elements. Additional losses arise due to the great recovery charges and high reverse currents when rapid voltage and current changes occur.
The dissipated power occurring at higher frequencies, cause high-frequency and fast current and voltage changes which generate a large amount of radio interference which can be reduced to a reasonable degree only with complicated filters.
The fact that the occurring, fast voltage and current changes in traditional switched power packs place limits on an increase in the switching frequency resulted in a new principle of DC conversion which are referred to a resonant converters.
The publication of the Unitrode Company by R. Patel and R. Adair, "High Frequency Series Resonant Power Supply--Design Review," Unitrode Power Supply Design Seminar, Unitrode Publication No. SEM-300, Topic No. 5 discloses a power pack comprising a series resonant converter. The basic idea disclosed in this arrangement comprises a primary circuit of the transformer which is used therein as a LC resonant circuit. This is charged for a specific time as required and is subsequently discharged for the same time, whereby the charge and discharge times are specifically matched to the component parts employed such that the resonant condition for the series resonant circuit is met given nominal load of the secondary circuit.
As known, the complex resistance, ie. the reactive impedance of the circuit, is equal to zero resonance in the series resonant circuit. The current, also called resonance current, is in phase with the voltage of the resonant circuit and its amplitude reaches its maximum value which is limited only by the actual resistance of the series resonant circuit.
In more detail, when charging the resonant circuit, the current curve can be represented by a sine half-wave which, when the resonant circuit is discharged, is followed by a second sine half-wave having an opposite polarity, whereby a complete sine wave of the current occurs.
More detailed knowledge about such equalization and calculations are disclosed in the publications, such as, for example the Hand Book "Elektro Technik", Volume 1, Fundamentals 1968.
Fast current changes in the switched power pack are consequently avoided by using a series resonant circuit operated at resonance.
Considering the principal of a series resonant converter, such as the circuit arrangement shown in the afore-mentioned publication of the Unitrode Company, it is seen that the effective inductance of the series resonant circuit is composed of the inductance of the transformer at the primary side and of the inductance of a choke. This arrangement was selected in order to keep the influence of the inductance of the transformer dependent on the load of the secondary circuit as low as possible on the effective inductance of the series resonant circuit. To this end, the inductance of the choke must be as high as possible in comparison to that of the transformer. This, however, is opposed by the fact that the series connection of the choke and of the transformer is equivalent to an inductive voltage divider which makes the voltage available at the transformer dependent on the current and which causes decreases in efficiency due to the magnetization losses arising in the choke.
Since the inductance of the choke cannot be arbitrarily increased for the above reasons, the natural frequency in fact coincides with the constant drive frequency in a resonant circuit formed in this way only in a narrow range of the load of the transformer at the secondary side. When this narrow range which is usually designed as a nominal load range is left, asymmetrical over-voltages occur at the energy storage means and over-voltages occur at the switching elements. In order to protect these, the voltage must be limited at the capacitor of the resonant circuit with diodes, and a part of the transformable power is lost in the diodes. Further, intermittence of the resonant current occur outside the resonant point, and high losses in the switching elements occur as a result.